Ionic transistor using ion exchange membranes

Abstract

Ionic transistors can be used to modulate ionic current in a way that is analogous to their electronic counterparts. An ionic transistor can reversibly change its ionic conduction to control ionic current by injecting electrical charges. To facilitate its applications in biomedical devices (e.g., controlled drug delivery, rectification of ionic current, and signal processing), an ionic transistor should maintain high performance of ionic current control within physiological solutions (e.g., 0.9% NaCl) for long durations. Here, we introduce an ionic transistor using cation and anion exchange membranes (CEM and AEM). It could impose a 10× impedance change in a channel filled with 0.9% NaCl solution and we observed a stable modulation of ionic current throughout a test of 1000 cycles of on/off switching of the ionic transistor.

Fig. 1|. The design and mechanism of the ionic transistor.

a, The structure of the ionic transistor with CEM and AEM. I_c is the ionic current that controls the transistor. I_o is the ionic current that is controlled by the transistor. L, w, and d are the dimensions of the paper channel. b,c, The ion flow during the application of I_c. The thickness of I_o arrow indicates the amplitude of I_o. “X” denotes the blocking effect of the IEMs. d, The set-up that was used to characterize the impedance of the ionic transistor. I_c was delivered via two carbon cloth electrodes positioned in the reservoirs (blue) filled with saline. The impedance of the paper channel was measured using the four-electrode method via Ag/AgCl electrodes. I_test was a 30μA sinusoidal current delivered to pass along the paper channel. The voltage drop along the paper channel was measured. e, The set-up for passing stable ~1mA I_o that was controlled with the ionic transistor. I_c was delivered via two carbon cloth electrodes. I_o was delivered via two stainless steel electrodes positioned in two syringes filled with 0.9% saline that were in contact with the paper channel. A sense resistor (Rs=2.1KΩ) was used to measure I_o.

Fig. 2|. The time response of the ionic transistor switching from on (low impedance) to off (high impedance).

a, Experimental results of the impedance of the paper channel (d=0.04, 0.1 and 0.2mm) that was normalized with its initial impedance at t=0 (N=3). V_c =+40V was applied to deplete ions. b, Simulation results of the redistribution of ions in the paper channel with d=0.04mm (left) and 0.2mm (right) at 30 seconds of ion depletion with V_c=+40V. The red and white colours represent low and high ion concentrations respectively. c, The time response of the normalized channel impedance with different V_c (+10, +20 and +40V) (N=3). d, The normalized channel impedance at 30 seconds (black) and the average time constant (red) of on-to-off switching with different V_c (N=3). The black dashed line represents 10x impedance increase. e, The time response of the channel impedance with different I_c (0, +0.04, +0.08 and +0.12mA) applied after on-to-off switching (black line) with V_c=+40V (N=3). The red bar on the top describes the timeline of the control input.

Fig. 3|. The impedance of the paper channel when the ionic transistor was switched on/off cyclically.

a, The example waveform of the impedance when I_c=0mA during off-to-on switching (from 100s to 140s in the first cycle). The low and high impedances in each cycle are blue and red respectively. b, The low and high impedances, and their contrast ratio (high/low impedance, red) over 10 cycles when I_c=0mA during off-to-on switching (N=3). c, The low and high impedances, and the contrast ratio over 5 cycles when Q_enr=Q_dep in each cycle (N=3). d, The contrast ratio over cycles when Q_enr = 0%, 98% and 100% of Q_dep in each cycle (N=3).

Fig. 4|. The controlled ionic current (I_o) under the cyclic modulation with the ionic transistor.

a, The ionic direct current (iDC) I_o that was cyclically modulated with the transistor. The red bar on the top describes the control input. The reverse I_c=−1.16mA was calculated with 98% of Q_dep. The blue and red lines represent the amplitudes of the I_o after the transistor was switched on and off respectively. b, The contrast ratio of the cyclically modulated iDC over 1000 cycles. (N=3) c, The ionic alternating current (iAC) that was cyclically modulated with the transistor. The red envelope of the black waveform indicates the amplitude of the modulated iAC. d, Contrast ratio (black line) for iAC delivered at different frequencies. (N=3) The blue line is the AC capacitive leak associated with the test platform.